1. Field of the Invention
The present invention relates generally to crucibles and crucible susceptors and methods of making the same and more particularly to such crucibles and susceptors used to melt silicon in a furnace.
2. Description of the Related Art
There are several methods for making single crystal materials. One such method—the Czochralski (CZ) process—has been widely adopted for producing single crystal materials for semiconductor applications such as integrated circuits and solar cells. In the CZ process, molten silicon is contained within a vessel, and a tip of a single-crystalline seed crystal is dipped into the molten silicon. The seed crystal is slowly pulled from the melt while being rotated. As a result, the melted silicon forms a single crystal silicon ingot around the seed crystal.
A fused quartz crucible is one vessel commonly employed to contain molten silicon in the CZ process. Such a crucible is generally in the shape of a bowl and is held by a susceptor during the melting and pulling of the crystal. The susceptor is typically made of a carbon element such as graphite or is a carbon composite.
At the start of the CZ process, a silica glass crucible that contains a solid silicon charge is placed inside the susceptor at room temperature. Although the susceptor is also bowl shaped to hold and support the crucible, there is a small gap between most of the outer surface of the crucible and the inner surface of the susceptor to permit the crucible to be inserted into the susceptor. Next, the susceptor is placed in a furnace, and heaters surrounding the susceptor are activated to begin melting the silicon, which melts at 1414 degrees C.
Heating continues until the silicon in the crucible is fully melted, i.e., in liquid form. The surface of the melted silicon is well below the top of the crucible. (A plane containing the top surface of the melted silicon is referred to herein as the melt plane.) As a result of the high temperature, the crucible softens. In addition, the weight of the silicon melt presses the outer surface of the crucible below the melt plane firmly against the inner surface of the susceptor. At this stage, there is virtually no gap between the crucible and the susceptor below the melt plane. Above the melt plane, however, the gap remains because the temperature is lower and because the melted silicon is not urging the crucible outwardly.
Once the silicon is completely melted, the seed crystal is dipped into the melt and slowly pulled therefrom while being rotated. It is important to control the rate of withdrawal because changes in the rate produce changes in the diameter of the single crystal ingot that is formed around the seed crystal. If the crystal structure is sufficiently disturbed, it may be necessary to start again.
In the past it was necessary to keep the temperature just high enough to maintain the silicon in its melted condition. Any higher temperatures tended to cause impurities from the crucible or from the refractory materials to be released into the atmosphere or, in the cased of the crucible, directly into the silicon melt.
More recently, improvements in manufacturing of the crucible, susceptor, and other refractory materials have improved their purity. This permits operating at somewhat higher temperatures without the defects associated with less pure materials. As a result, processors have increased the furnace temperature during the CZ process, especially during the melt down process, to speed up throughput. This is especially true for manufacturers of solar cells, which can be less chemically pure than the silicon used for integrated circuits.
But unexpected losses in the crystal structure were encountered at these higher temperatures, starting at between about 1580 and 1620 degrees C., even when using crucibles and refractory materials having a high purity. The assignee of the present application investigated these losses and discovered that at these higher temperatures, gas is evolved from the outer surface of the silicon crucible below the melt plane. This gas may include silicon oxide, carbon monoxide, and/or carbon dioxide. Because the outer crucible surface is effectively sealed against the inner susceptor surface, the evolved gas blows up the crucible wall into the melt, which changes the position of the melt surface. Although this elevation of the melt plane is most prevalent during the melting process, the elevated melt level may change while the ingot is pulled. This change produces an effect similar to changing the pull rate, and in some cases, crystal structure of the ingot is lost.
This problem is accentuated for larger crucibles. In such crucibles, the heaters, which surround the susceptor, must be set to generate a high enough temperature to maintain the silicon in a melted condition at the center of the crucible. It is apparent that the temperature drop between the outermost portions of a larger crucible to the center thereof is larger than for a smaller crucible. This results in higher heating of the periphery of the larger crucibles, which in turn increases the likelihood that the wall of the crucible will blow inwardly as described above.